Fracture-permeation is a new technique developed in our laboratory to probe the compactness of the cytomatrices in glutaraldehyde-fixed cells. In fracture-permeation, macromolecules of known size are used to assess the mesh of a chemically-fixed matrix. Fracture-permeation of rat cardiac muscle revealed details of the organization of mitochondria, in particular variations in the compactness of the mitochondrial matrix. In other experiments we used fracture-permeation with cationized ferritin to assess the intermolecular spacing in the extracellular matrices of rat kidney glomeruli: the basement membrane and the mesalgyal matrix. We have also devised a variant of the above permeation technique where glutaraldehyde-fixed tissue was minced instead of freeze- fractured. This alternative method duplicated the results of the fracture-permeation technique establishing the distribution of intermolecular spaces in sarcomeres of striated muscle and the structure of the mitochondrial matrix.